(1) Field of the Invention
The present invention relates to a method and apparatus for measuring coherent crosstalk light, which is a cause of deterioration in signal quality in an optical transmission system.
(2) Description of the Related Art
In the transmission devices and transmission paths in optical transmission systems, there are several junctions where optical fibers connect to each other. These junctions are in the form of an optical connector or a splice (fusion splice), and at these junctions, part of the signal light is reflected (through Fresnel reflection, for example) due for example to gaps in the engagement part of the optical connector or contamination at the end faces of the connector. When there are several of these reflection points, then as shown for example in FIG. 22, a portion of the signal light is reflected repeatedly at the various reflection points, and the component in this multiple reflected light which travels in the same direction as the signal light ultimately becomes coherent crosstalk (hereunder referred to as CXT) light. CXT light generates beat noise in the signal light in the receiver, causing deterioration in signal quality such as bit error rate (BER), for example. Typically, the amount of CXT light generated (referred to as the CXT amount below) is defined by the following formula (1), using the power PTR of the primary signal light transmitted through the plurality of reflection points, and the power PXT of the multiple reflected light (CXT light) which travels in the same direction as the primary signal light.CXT amount=PXT/PTR  (1)
FIG. 23 shows an example of calculating the relationship of the signal quality (in terms of the transmission penalty) with respect to the CXT amount. From FIG. 23 it is apparent that when the CXT amount increases, the transmission penalty increases rapidly, and the signal quality deteriorates.
Accordingly, if the CXT light which is actually being generated can be measured, it is possible to find out which location in an optical transmission system with a large number of optical parts is having an adverse effect, and provide a prompt and easy solution (for example cleaning the reflection points or replacing the connector). However, as is also apparent from the optical spectra shown on the right side of FIG. 22, the optical frequency of the CXT light is exactly the same as that of the transmission light, which means that both types of light overlap completely in the optical spectrum. Therefore, it is impossible to differentiate between them, and consequently, it has been difficult to directly measure only the CXT light.
Consequently, in conventional technology, instead of measuring the CXT light directly, it is typically most common for measurement based on reflected light detection such as optical time domain reflectometry (OTDR) to be performed with an object of measuring the location of reflection points and the reflected amount. In this method, the position of reflection points and the amount of reflection are calculated by detecting the light which is actually returned from the reflection points. However, the measuring apparatuses which are currently most often used to implement this method have a construction which is inherently suitable for measuring the reflection points within transmission paths with lengths of up to several dozen kilometers, but are not well suited to use in locations where the optical paths are short, for example in the measurement of reflection points in optical components within an optical transmission device.
On the other hand, as a method which has few limitations in terms of measurement distance resolution, technology which enables accurate measurement of reflection light power, in reflectometry (OCDR) using synthesis of the optical coherence function, by pulsing the output light, for example, has been proposed (see Japanese Unexamined Patent Publication No. 10-148596, for example). Furthermore, as a method of detecting the reflection points in the various optical components in a device, technology where the reflection light generated in the device is detected for example by respectively providing reflection monitors at the input ports and output ports of each component, has been proposed (see Japanese Unexamined Patent Publication No. 2003-51785, for example).
However, such conventional technology presents a problem in that in theory, when detecting light returning from the reflection points, if a unidirectional optical component (for example an optical isolator) is positioned in the object of measurement, the reflected light is cut out at that point and cannot be measured.
Furthermore, although the conventional technology described above has sufficiently high distance resolution for specifying the reflection points, because accurately measuring the reflectance is not an object of this technology, it is not suited as a device for measuring the amount of CXT light. Hypothetically, even if the reflectance could be measured accurately by applying the conventional technology, the only way to determine the actual CXT amount is to estimate the CXT amount by calculating it indirectly based on the reflectance. In other words, technology which measures the ratio of the power of the CXT light with respect to the power of the primary signal light which passes through a plurality of reflection points is yet to be realized.